Hospital trusts will each be asked to check the case notes of about 100 patients who died while undergoing treatment to calculate how many were lost "needlessly" under plans being put forward by an academic tasked with working out how many NHS deaths are avoidable every year.

Nick Black, professor of health services research at the London School of Hygiene and Tropical Medicine, was asked by NHS England in July to investigate the relationship between calculated "excess mortality rates" and preventable deaths in hospitals.

About 215,000 patients die every year while in the care of the NHS and a paper by Black last year examined 10 hospital trusts in detail, concluding that one patient in 10 is affected by potentially serious medical errors, with half dying as a result.

At that rate, 12,000 adults would be dying needlessly in the NHS in England.

Black was hired after a review by the NHS medical director, Sir Bruce Keogh, into the quality of care at 14 hospital trusts with the highest mortality rates.

Keogh stated that it was not possible to work out how many potentially avoidable deaths there may have been at a hospital from the mortality indexes in use, known as "hospital standardised mortality ratio (HMSR). But by next April a new scheme will be in place based on hospitals combing through the case notes of 20,000 patient deaths – about 120 chosen randomly in each trust – to calculate the "preventable death rate" in the NHS.

This will then be checked by an independent team which will randomly select 2,000 deaths – 10% of the total – to ensure there is no "gaming" of the system, a frequent charge made against hospitals who hire computer experts to "code" their mortality data in such a way that flatters their mortality rates.

Black said that the idea was not to persecute any individuals but it identify "organisational failure". He said: "What we have found is that many preventable deaths are occurring in elderly people with multiple pathologies. These are patients with a matter of weeks to live. We want to identify mistakes contributing to these deaths."

Black told the Guardian that he "was not a fan" of HSMR. "I don't know any academics who think HSMRs are a valid form of rating hospitals. They are a nonsense".

The academic says his own work was based on careful examination of the records of 1,000 patients. As a first step Black will conduct the largest study of errors in British hospitals – and expand his work to 34 NHS trusts – to check that his finding that 5% of patients died needlessly. Earlier this summer newspapers reported that their had been 13,000 "excess" deaths in just 14 hospital poor-performing trusts since 2005 – a figure that came from data produced by Professor Sir Brian Jarman of the Dr Foster unit at Imperial College, who devised the HSMR. He defended his index and pointed out countries such as Canada, the Netherlands, Denmark and Scotland all used mortality rates.

He said the Francis report into the Mid Staffs scandal had endorsed the idea they were a useful "smoke alarm". He warned that Black's proposals would be "expensive and time-consuming and probably not produce a very different outcome" from his own work.

"When I worked on the Bristol inquiry into deaths at a child heart surgical unit we had 80 cases to consider and it took 49 trained clinicians months to come to their conclusions which were not wildly different to what the excess deaths data showed," said Jarman.

Keogh said that Black's proposals would be considered as part of his review. "We want to use this as a pilot to roll out nationally to improve quality. We want to have a measure of avoidable deaths in the NHS. It's important to know that doctors' work will be examined by external authorities."